The present invention relates to a new and improved method of, and apparatus for, the determination of the time dependency of the length of a fissure or crack in a test specimen or piece during a fracture test, wherein an electrically conductive, areal or surface-like element is attached in an insulated fashion at the test specimen such that it likewise tears at the region of the fissure during the fracture test, and wherein there is determined the electrical resistance between two measuring locations of the element, the measuring locations being selected in such a manner that the measuring current travels between the measuring locations through the region of the fissure, so that the resistance increases as a function of the length of the fissure.
During the determination of the breaking or rupture strength, for instance the tear strength of materials, there is oftentimes present the problem of measuring the length of the fissure or fracture which is forming in the test specimen during the entire fracture test. A state-of-the-art method employs for this purpose an electrically conductive element which however is insulated at one side. This element possesses a number of juxtapositioned, parallely extending, conductive strips which at least at both ends thereof are all conductively connected with one another. Prior to carrying out the actual fracture test, the element is bonded to the test specimen in such a manner that the strips extend transversely over the line at which later there is formed the fissure or crack. The element is connected at both sides of the fissure line with a measuring device for measuring the resistance. If a fissure is formed in the test body during the fracture test, then the strips of the element are also successively torn. Upon breakage of each strip, the resistance of the element is somewhat increased in stages. This resistance then constitutes a measure for the length of the fissure or crack.
This prior art method is associated with the drawback that the fissure length can only be discontinuously measured, and the resolution of the length is determined by the spacing of two neighboring strips. This spacing cannot be chosen to be randomly small, since otherwise the element no longer can be successfully bonded to the test specimen or body. Hence, the measurement, at best, can be accomplished with an accuracy of several tenths of a millimeter. This is then especially disturbing if it is desired to control the force exerted by the testing machine upon the test specimen as a function of the fissure length.